Despite extensive study, the contributions of the superficial layers of the superior colliculus to visual function remain obscure. An important recent insight, however, has been that the superficial grey layer (SCS) comprises multiple sublaminae, differing from one another in retinal and non-retinal inputs, histochemistry, unit response properties and efferent projections. These findings raise the possibility that several distinct visual "channels" may course through the SGS, each originating in particular ganglion-cell classes, undergoing intracollicular modulation by distinct afferent and neurotransmitter systems, and exerting its influence on particular target structures through the output of unique collicular cell types. The proposed studies will test this hypothesis directly for single postulated collicular channel, the retino-tecto-geniculate pathway, originating in retinal W-cells, synapsing on "tectogeniculate cells" in the upper SGS, and reaching the deep C-laminae of the lateral geniculate nucleus. Tract-tracing, conduction-velocity and receptive-field analyses will be used to: l) identify which subclasses of retianl W-cells drive tectogeniculate neurons; 2) specify the functional properties of identified tectogeniculate cells; and 3) determine the contribution of cortical and subcortical afferents to the tectogeniculate output signal. The findings of these studies will clarify the role of ascending tectofugal pathways in vision, the functional organization of well-defined geniculocortical subsystem, and the significance of heterogeneity with the poorly understood W-cell class.